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Blood Viscosity Defined

Blood viscosity is the thickness and stickiness of blood. It is a direct measure of the ability of blood to flow through the vessels. It is also a key screening test that measures how much friction the blood causes against the vessels, how hard the heart has to work to pump blood, and how much oxygen is delivered to organs and tissues. Importantly, high blood viscosity is easily modifiable with safe lifestyle-based interventions.

Blood viscosity is defined as the inherent resistance of blood to flow. Normal adult blood viscosity is 40/100, which is read as “forty over one hundred” and reported in units of millipoise.

Increased blood viscosity is the only biological parameter that has been linked with all of the other major cardiovascular risk factors, including high blood pressure, elevated LDL cholesterol, low HDL, type-II diabetes, metabolic syndrome, obesity, smoking, age, and male gender [1].

One might be tempted to think of blood simply as a transportation system for the body’s organs or as a hydrating fluid. But it is important to remember that the blood is an organ in and of itself: a very large collection of living cells that interact with one another. The blood is 3-4 times larger by volume than the brain, and 2-3 times larger than the liver.

Blood is a vigorous organ insofar as it behaves as a non-Newtonian fluid, which means that its viscosity changes as a function of shear rate. Think of shear rate as velocity. When blood moves quickly as in peak-systole, it is physically thinner; when it moves slowly during end-diastole, it is thicker and stickier. This is because red cells aggregate. The phenomenon is known as the shear-thinning, non-Newtonian behavior of whole blood [2].

Human blood viscosity varies dynamically from high shear to low shear during each cardiac cycle. At systole (high shear rate), blood is thinner, while at diastole (low shear rate), blood is 2-5 times thicker.

The largest blood viscosity study ever conducted was part of the Edinburgh Artery Study in the 1990s, which followed a random population of 1,592 middle-aged adults for a mean of 5 years. It showed that blood viscosity, after adjustment for age and gender, was significantly higher in patients experiencing heart attacks and strokes than those who did not (p = 0.0003). The 20% of the individuals with the highest viscosity had 55% of the heart attacks and strokes during the 5-year period. In contrast, only 4% of those in the lowest viscosity group had any significant events.

What is interesting is that these findings were based solely on measuring systolic blood viscosity (that is, high shear rate viscosity), where the variation range is very narrow. Even so, the link between systolic blood viscosity and cardiovascular diease events was statistically as strong as the links between diastolic blood pressure and LDL cholesterol on one hand, and cardiovascular events on the other. The association between systolic viscosity and cardiovascular events was stronger than that between smoking and cardiovascular events [3].

In a prospective study, 331 middle-aged men with high blood pressure were followed for up to 12 years after measuring diastolic blood viscosity (i.e., low shear rate viscosity). The subjects were divided into three groups according to viscosity levels: those in the highest viscosity group had more than three times more cardiovascular events than those in the lowest viscosity group (hazard ratio = 3.42, 95% confidence interval = 1.4–8.4, p = 0.006), [4].

In a study of 128 obese people (BMI > 28 kg/m2) and 90 non-obese healthy controls, diastolic blood viscosity was 15% higher in obese vs. non-obese patients [5]. Numerous other studies have also shown that type-II diabetics have higher systolic and diastolic viscosity than healthy non-diabetic people. Patients with metabolic syndrome have higher viscosity than those without, and viscosity scores can predict incident diabetes in initially non-diabetic adults.

Many studies have linked cholesterol with blood viscosity; LDL is consistently associated with higher blood viscosity, while HDL is associated with lower viscosity [6]. Smoking has been shown to cause blood viscosity to surge by as much as 20%, depending on the degree of cigarette use.